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Provide an intermediate response when a WSGI application slow to respond

Project description

Status And License

Wait For It is distributed under an MIT-style license. It is by Ian Bicking. Questions can go to the Paste mailing list; bugs can be reported in the Paste Bug Tracker.

What It Does

Wait For It gives users a timely response even when the underlying WSGI application isn’t very timely. Wait For It does this by launching an extra thread for each incoming request. If the wrapped application responds in a timely manner (before time_limit) the response is simply passed on.

If the response doesn’t come back before the time limit, the user is given a response page that asks them to wait. The response page contains Javascript that will re-check the status of the page, and when the page is ready it will reload the page.

Applications can provide feedback to users by looking for environ['waitforit.progress'] and putting information in there. In particular "message" contains an HTML message for the user (e.g., if you want to show what step the long-running application is working on). "percent" is used for a progress bar; it should be a number from 0 to 100. The value in progress is sent via JSON, so you should use only strings (preferably unicode), integers, floats, dictionaries (with string keys) and lists.

Using It

Wrap you application like this:

from waitforit import WaitForIt

fast_app = WaitForIt(slow_app, time_limit=5, poll_time=10)

Then if slow_app takes longer than 5 seconds, a transition page is shown. The transition page checks to see if the response is done every 10 seconds. (The default values for both parameters is 10 seconds.)

In your application you can do:

def slow_app(environ, start_response):
    progress = environ.get('waitforit.progress', {})
    for i in range(100):
        progress['message'] = 'I have completed %s out of 100 tasks' % i
        progress['percent'] = i
    ... handle response ...

Also if some requests should be handled synchronously (even if they may be slow) you can indicate that with:

def slow_app(environ, start_response):
    progress = environ.get('waitforit.progress', {})
    if you_should_really_wait_for_this_to_return(environ):
        progress['synchronous'] = True

That is, setting environ['waitforit.progress']['synchronous'] = True, if done in a timely way, will keep WaitForIt from intercepting the request.

Download & Installation

You can install with easy_install WaitForIt. You can also install the most recent code from trunk with easy_install WaitForIt==dev. You can see the most recent code in the svn repository, or check it out like:

svn co WaitForIt


To see a simple example, see waitforit.testapp. You can run it like python waitforit/ (or if you are using Python 2.5, python -m waitforit.testapp).

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